Can you cast a sorcery with overload during your main phase only or at other times too?

Overload is a keyword that lets you pay an alternative cost to change a spell's text so it affects all valid targets instead of just one. Crucially, overload does not grant any special timing permissions. The spell's card type still determines when it can be cast (CR 702.97a).

Because a spell with overload on a sorcery is still a sorcery, you can only cast it during your own main phase when the stack is empty and you have priority — the standard sorcery-speed restriction (CR 307.1). Paying the overload cost changes the targeting mode, not the timing rules.

If overload appeared on an instant instead, you could cast it at instant speed as normal, again because the card type governs timing, not the overload keyword itself.

Concrete example: You control Mizzium Mortars (a sorcery with overload). You can cast it with its overload cost only during your main phase when the stack is empty. You cannot flash it in during an opponent's combat step just because you're paying the overload cost — the sorcery timing restriction still applies.

Relevant rules: CR 307.1 (sorcery timing), CR 702.97a (overload definition), CR 702.97b (overload only changes targeting text and cost, grants no other abilities).